While walking in the woods with my friend Gary Chastagner, we stumbled upon some old tree stumps. The stumps had so many rings we had to use our magnifying glasses to see them all.

“It’s a timeline,” said Chastagner, examining the stump. I thought it might make a nice scratch post, too.

“There is usually a single ring added each year because of the transition between the growth of the previous year and the new growth,” he said.

He pointed out that the growth rings were different colors. Some were lighter and others darker. Some were thicker while others were thinner. All these differences tell us about how the tree grew throughout its life.

Chastagner is a scientist at Washington State University and helps plants, particularly Christmas trees, stay healthy.

He said some trees actually don’t appear to have rings at all. Other trees will grow thousands of rings, if they live long enough.

In the redwood forests of California, the trunks of some trees have grown so wide it would take 25 kids holding hands to circle one. In fact, some people have found old redwood stumps with more than 3,000 rings. They’re ancient.

There’s one particular part of the tree that forms these rings each year or in some species, twice a year. In the spring, if you peel back a tree’s bark, you’ll find a slippery surface.

“That’s the cambium and it lays down the new woody cells each year,” Chastagner said.

The cells are like building blocks that produce new layers of wood. In spring, the tree grows pretty fast. The fast growth makes larger cells that form a ring with lighter color. In summer, the tree is often stressed from lack of water or heat. The cells are much smaller and form a ring that is darker.

While the color tells us what time of year the rings formed, the size and shape of the rings tell us a few other things about the tree, too.

“You can kind of look at stresses that the tree was growing under,” Chastagner said.

Often, the tree stress is related to the weather conditions. In fact, tree rings can tell us a lot about weather patterns over the years, which we call climate. In a way, they’ve been recording weather conditions longer than humans.

If the tree has enough water, sun, and space to grow, the rings will be thicker. But if weather conditions aren’t so great, a tree might struggle for resources and grow thinner rings.

By comparing different sized rings and the tree’s age, scientists can understand more about droughts, severe storms, attacks by insects or disease, and natural disasters that happened long ago.

Trees are some of the oldest living organisms on our planet, Chastagner reminded me, as we explored the forest. While most of us spend our time admiring the outside of towering trees, we know that on the inside, trees are also leaving an amazing story of their life on Earth.